1. Field of the Invention
The present invention relates to an optical pickup device employing, for focus control, the astigmatism method that includes causing a laser beam, emitted from a laser beam source, to be reflected by a plane-parallel plate beam splitter so as to guide the beam to an objective lens, causing the objective lens to focus the laser beam so as to irradiate the beam into a signal recording medium, causing the laser beam, reflected and returned from the signal recording medium, to transmit through the beam splitter so as to guide the beam to a photodetector, and imparting astigmatism to the laser beam received by the photodetector.
2. Description of the Related Art
An optical pickup device for optically recording/playing back a signal to/from a signal recording medium such as a CD or DVD optical disk using a laser beam employs one of the three-beam method, the push-pull method, and the phase difference method or an applied method thereof as the tracking control method adapted to cause a laser beam, irradiated into the signal recording medium, to follow a signal track on the signal recording medium to ensure compatibility with the standard system of the signal recording medium.
On the other hand, the astigmatism method is normally used for focus control that is intended to focus the laser beam, irradiated into the signal recording medium, to a signal surface of the signal recording medium.
For example, some optical pickup devices compatible with various types of recordable/playable CDs, DVD-ROMs and DVDs±R/RW employ, as a tracking control method, the phase difference method for DVD-ROMs and the differential push-pull method for DVDs±R/RW and CDs and, as a focus control method, the astigmatism method for various types of recordable/playable CDs and the differential astigmatism method for various types of recordable/playable DVDS.
The differential push-pull method, the differential astigmatism method and the three-beam method all require three beams. Therefore, the optical pickup device employing any one of the above methods uses a photodetector provided with a diffraction grating adapted to diffract and form a zero-order diffracted light beam and positive and negative first-order diffracted light beams, as universally known, so as to separate a laser beam emitted from a semiconductor laser into three beams, and light receiving regions respectively adapted to receive reflected light beams of the three beams reflected by the signal recording medium.
FIG. 2 illustrates an example of a light receiving surface of the photodetector compatible with the optical pickup device exemplified above, and the tracking control system and the focus control system will be described with reference to this figure.
Of the three beams irradiated into the signal recording medium, the main spot of the zero-order diffracted light beam is arranged at the center on the signal track, whereas the sub-spots of the positive and negative first-order diffracted light beams are each arranged at the center between the signal track with the main spot and either of the adjacent signal tracks in different directions from each other.
The light receiving surface of the photodetector has three light receiving regions arranged vertically one on top of the other, namely, a main light receiving region 21 and sub-light receiving regions 22 and 23, each made up of four segments as a result of cross-shaped division into four parts by two dividing lines that intersect each other at right angle. The main light receiving region 21 and the sub-light receiving regions 22 and 23 respectively receive the zero-order diffracted light beam and the positive and negative first-order diffracted light beams reflected by the signal recording medium. In this case, one of the dividing lines or dividing lines 21A, 22A and 23A of the main light receiving region 21 and the sub-light receiving regions 22 and 23 are provided in the direction of the signal tracks of the signal recording medium in consideration of the tracking control system used.
It is to be noted that, in FIG. 2, individual light reception outputs obtained from the individual segments making up the main light receiving region 21 and the sub-light receiving regions 22 and 23 are designated as ‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’ and ‘l’, respectively.
The phase difference method employed for tracking control of DVD-ROM playback detects the phase difference between sum signals (a+c) and (b+d) obtained by adding the light reception outputs of the segments diagonal to each other in the main light receiving region 21 or the phase difference between the light reception outputs ‘a’ and ‘d’ or ‘b’ and ‘c’ of the given adjacent segments divided by a dividing line 21A of the main light receiving region 21 that runs in the direction of the signal tracks See, for example, Japanese Patent Application Laid-Open Publication No. 2000-276742.
The differential push-pull method employed for tracking control of DVD±R/RW recording and CD recording and playback calculates a main push-pull signal that is a difference signal between sum signals obtained by adding the light reception outputs of the two segments each divided by the dividing line 21A of the main light receiving region 21 that runs in the direction of the signal tracks, calculates first and second sub-push-pull signals that are respectively difference signals between sum signals obtained by adding the light reception outputs of the two segments each divided by the dividing line 22A and 23A of the sub-light receiving regions 22 and 23 that run in the direction of the signal tracks, adjusts a combined sub-push-pull signal, obtained by combining the first and second sub-push-pull signals, with a gain (G1) in conformity with the main push-pull signal, and calculates the difference between the main push-pull signal and the combined sub-push-pull signal to obtain a tracking error signal having the calculation formula “a−b−c+d−G1(e−f−g+h+i−j−k+l)” See, for example, Japanese Patent Application Laid-Open Publication No. 11-296875.
The astigmatism method employed for focus control of various types of recordable/playable CDs imparts astigmatism to the reflected light beams reflected by the signal recording medium such that astigmatism occurs at 45° angle relative to the dividing lines of the main light receiving region 21 and the sub-light receiving regions 22 and 23, adds the light reception outputs of the segments diagonal to each other in the main light receiving region 21 to obtain two sum signals, and calculates the difference between the two sum signals to obtain a focus error signal that is a main astigmatism signal having the calculation formula “a+c−(b+d).”
The differential astigmatism method employed for focus control of various types of recordable/playable DVDs, obtains a sub-astigmatism signal of each of the sub-light receiving regions 22 and 23 as with the main light receiving region 21 in the above astigmatism method, adjusts the sub-astigmatism signals of the sub-light receiving regions 22 and 23 with a gain (G2) in conformity with the main astigmatism signal, and adds the sub-astigmatism signals to the main astigmatism signal to obtain a focus error signal having the calculation formula “a+c−(b+d)+G2(e−f+g−h+i−j+k−l)” See, for example, Japanese Patent Application Laid-Open Publication No. 11-296875.
Incidentally, astigmatism required for the astigmatism or differential astigmatism method is generated by providing cylindrical, toric or toroidal anamorphic lenses with different focal distances in two directions perpendicular to the optical axis that intersect each other at right angle in the optical path guiding the reflected light beams reflected from the signal recording medium, or by providing a plane-parallel plate in tilted position relative to the plane perpendicular to the optical axis.
In an optical pickup device using a plane-parallel plate beam splitter to separate the laser beam source and the photodetector in different optical paths, the reflected light beams reflected from the signal recording medium transmit through the beam splitter to be guided into the photodetector as a result of providing the photodetector in the optical path of the beam splitter. Therefore, the beam splitter is used to generate and impart astigmatism for focus control to the light beams received by the photodetector.
A plane-parallel plate beam splitter is employed in a number of optical pickup devices for its higher potential for cost reduction as compared to a prism beam splitter.
Incidentally, when an optical pickup device employs the phase difference or push-pull method (including the differential push-pull method) for tracking control, one of the dividing lines must be provided in the direction of signal tracks in the light receiving regions of the photodetector (main- and sub-light receiving regions).
If dividing lines of the photodetector's light receiving regions must be provided in consideration of the signal track direction as described above, the direction of astigmatism generated by the plane-parallel plate beam splitter is often inappropriate relative to the dividing lines of the photodetector's light receiving regions and for the astigmatism method due to design restrictions such as the optical arrangement of the optical pickup device.
In such a case, a prism beam splitter is normally used, and a different optical element, adapted to generate astigmatism, is provided in the optical path leading to the photodetector.
Alternatively, an optical element is provided in the optical path leading to the photodetector to generate astigmatism after the cancellation of astigmatism generated by the plane-parallel plate beam splitter. See Japanese Patent Application Examined Publication (KOKOKU) No. 06-77332.
Incidentally, in the optical pickup device described in Japanese Patent Application Examined Publication (KOKOKU) No. 06-77332, a single lens is given the functions to cancel astigmatism generated by a plane-parallel plate beam splitter and to newly generate astigmatism. This lens has, on one side, an anisotropic curvature adapted to generate astigmatism in opposite direction so as to cancel astigmatism, and a toric or toroidal surface having an anisotropic axis on the other side.
This requires a lens with special functionality. In addition, the relationship is fixed between the direction of astigmatism adapted to cancel astigmatism generated by the plane-parallel plate beam splitter and that of astigmatism newly generated. Therefore, the directions of these astigmatisms cannot be adjusted. This makes it impossible to finely adjust the directions of astigmatisms to be generated in response to the photodetector.